1. Field of the Invention
This invention relates to apparatus for planar flow casting of metal ribbon, particularly ribbon of amorphous metal alloys.
2. Description of the Prior Art
For purposes of the present invention, a ribbon is a slender body of substantially rectangular cross section whose transverse dimensions are much smaller than its length.
There has long been recognized the need for processes that would permit manufacture of finished or semifinished products such as wire, ribbon or sheet directly from the molten metal. Hubert et al. provided a review of such processes and classified then available techniques into the "melt spin process" and the "melt drag process" (Zeitschrift Fuer Metallkunde 64, 835 (1973)).
In the melt spin process, a jet of molten metal is cooled, either in free flight or by jetting it against a chill block, to obtain continuous filament. Both of these embodiments employ a pressurized orifice. There is also a melt spin process operating without an orifice, wherein molten metal is supplied to a jetforming device, such as a grooved spinning disk, to be expelled therefrom. Hubert et al. stated that the key to success in the melt spin process is to stabilize the liquid jet until it solidifies. Jets of molten metal are inherently unstable and have a strong tendency for droplet formation, because molten metal generally has low viscosity and high surface tension. Basic problems of jet stability have been discussed by Butler et al. in Fiber Science and Technology 5, 243 (1972).
In the melt drag process (see U.S. Pat. Nos. 3,522,836 and 3,605,863), molten metal is made to form a meniscus held by surface tension at the outlet of a nozzle. From this meniscus, molten metal is dragged onto a rotating cooled drum or continuous belt. Although this method avoids the difficulties of jet instability inherent in the melt spin process, it suffers from another shortcoming. Melt flow at the meniscus is severely limited. This, in turn, limits the chill surface speeds that provide continuous filament. Thus, the melt drag process may not provide sufficiently high molten metal cooling rates to permit production of amorphous metal strips. Such strips require quenching of certain molten alloys at a cooling rate of at least 10.sup.4 .degree. C. per second, more usually 10.sup.6 .degree. C. per second.
U.S. Pat. No. 4,142,571, issued Mar. 6, 1979 to Narasimhan, discloses inter alia apparatus for preparing continuous metal ribbon from the melt by the planar flow casting method. The disclosure of this patent is incorporated herein by reference. It provides apparatus which comprises a movable chill body, a slotted nozzle in communication with a reservoir for holding molten metal and means for effecting expulsion of the molten metal from the reservoir through the nozzle onto the moving chill surface.
The slotted nozzle is located in close proximity to the chill surface. Its slot is oriented perpendicular to the direction of movement of the chill surface and is defined by a pair of generally parallel lips, a first lip and a second lip, numbered in the direction of movement of the chill surface. The slot has a width, measured in the direction of movement of the chill surface, of about 0.2 to 1 mm. There is no limitation on the length of the slot (measured perpendicular to the direction of movement of the chill surface) other than the practical consideration that the slot should not be longer than the width of the chill surface. The length of the slot determines the width of the strip or sheet being cast.
The first lip has a width (measured in the direction of movement of the chill surface) at least equal to the width of the slot. The second lip has a width of about 1.5 to 3 times the width of the slot. The gap between the lips and the chill surface is at least about 0.1 times the width of the slot, but may be large enough to equal the width of the slot.
As the molten metal is forced onto the chill surface, a molten puddle is formed. The puddle extends a short distance upstream, forming a meniscus extending between the chill surface and the first lip of the nozzle. It has been discovered that a unique equilibrium position for the meniscus does not exist when the first lip consists of a single plane surface parallel to the chill surface as is disclosed in the prior art. Variations in the meniscus position (i.e., distance from the slot) give rise to undesirable variations in the dimensions of the cast product.
Although casting can be done under a vacuum (see, e.g., U.S. Pat. No. 4,154,283, issued May 15, 1979, to R. Ray et al.), casting in air is more convenient. However, turbulent air movement over the surface and sides of the molten metal may cause waviness and other undesirable surface variations.